Self-Cleaning Mechanical Filters Prove Reliable In Critical Industrial Applications
By Thomas Hamilton, Power Products & Services Co.The design improvements in self-cleaning mechanical water filters have expanded the range of applications in industrial facilities. These systems, no longer limited to perforated and wedgewire screens providing "rough" filtration, can effectively replace sand media filters, bag filters, and cartridge filters in many diverse applications. Fine weavewire screens (5 micron nominal and above) are successful in applications ranging from cooling tower sidestream filters to ultrapure / pre-R-O applications in industrial facilities around the world. One of the world's leading pulp & paper manufacturers tested the technology and have applied it to several areas in their manufacturing plant.
Canfor Pulp Limited Partnership (CPLP) is a global supplier of pulp and paper products with operations in the central interior of British Columbia. CPLP is the second largest North American and fourth largest global producer of market Northern Bleached Softwood Kraft Pulp, with an annual production of approximately 920,000 air-dry metric tons. They are also the leading producer of fully bleached, high performance Kraft Paper.
Figure 1: Canfor, Prince George Pulp Mill
In 2004, Canfor maintenance personnel at the Prince George pulp mill were researching water filtration products that could address an ongoing problem in the mill. The seal water supply system feeding pumps and agitators in the pulp mill was raw water from the Nechako River. The original equipment supplied, a sand media filter, had long been removed from service due to corrosion and maintenance issues. With raw river water feeding mechanical seals, gland packing and bearing cooling water, the plant spent a great deal of time flushing or replacing service water lines that plug with silt. Even more costly was the short service life, often only a month, of the packing and mechanical seals in the pumps.
Canfor contacted Power Products & Services Co, a distributor of Amiad Water Systems to determine if mechanical filters could resolve the issue. The system required continuous 24/7 operations and have the ability to handle the high-suspended solids present in the river during spring runoff, filtered to 80 micron absolute.
Based on the water quality parameters provided, a 4" SAF-4500 filter was sized for the 75 gpm flow rate. There was skepticism within Canfor's maintenance and engineering department, as the proposed filter was perceived as a similar design to other self-cleaning strainers that had not performed well at the mill. After a review of the design of the older technology vs. the proposed OEM's patented suction scanner cleaning technology, the plant inquired about a pilot filter test program to prove out the system; specifically the filters ability to maintain a clean screen. A skid mounted SAF-3000 pilot filter system (Fig. 2) was sent to the site to allow plant operators hands on experience with the system operation, and to ensure the filter would be properly sized for the high total suspended solids loads. After several months of operation during adverse water conditions, the plant was convinced the system would provide the service and reliability required for this critical application.
As insurance that the critical nature of the service would not be interrupted, an automatic bypass valve is installed so that in the event of a filter fault, the filter would be bypassed, thereby maintaining seal water to the equipment. The control signal opening the bypass valve also alerts control room operators of the filter fault condition. The installation of this system was completed in early 2005 and immediately improved the quality of the service water.
Based on the success of the first installation, Canfor purchased a second system, rated at 250 gpm flow with tighter, 50 micron screens in 2006. According to Mike Todd, the reliability engineer responsible for the projects, the improvement in water quality is clearly visible to the naked eye, and proven by filter inlet/outlet turbidity readings. While there has been improvement in the seal life, it is difficult to quantify, as the old pipes were not completely replaced. Some of the silt that built up in the system for years is still carried over to the seals. Now that the plant has an effective filtration system, replacing the old piping is an ongoing process that will be completed in the near future.
The filter design utilizes an epoxy coated carbon steel filter body with a four-ply screen constructed entirely of 316LSS. As can be seen in Figure 3, the working screen is "sandwiched" between heavier mesh screens to provide structural support. The wedgewire outer layer provides the structural rigidity for the assembly. The working screen provides filtration degrees from 10 micron up to 800 micron, designed specifically
for each application.
Filter operation is illustrated in the cutaway section of Figure 4. Dirty water first enters through a coarse perforated screen then into the fine screen. The primary purpose of the coarse screen is to protect the fine screen from large debris. The water travels through the fine screen from the inside out, trapping the fine materials on the inner screen surface. As the buildup occurs, the screen differential pressure increases. At approximately 7 psi differential, the filter backwash cycle is initiated.
When the DP switch is triggered, the exhaust valve is opened. The suction scanner cleaning device is a sealed pipe with nozzles that extend out close to the screen surface; (approximately 1/8 inch from the inner screen surface) its upper end is opened to the Flushing Chamber. Since the exhaust valve is piped to a sump, the differential between the supply pressure to the filter and atmospheric pressure at the sump creates a suction or vacuum effect at each of the suction scanner nozzles. This vacuum effect sucks the materials from the screen surface, through the scanner nozzles, and out the exhaust valve. The motor then rotates the suction scanner shaft at 20 rpm so that each of the 4 nozzles cleans 25% of the screen surface. The cleaning cycle lasts approximately 20 seconds, cleaning 100% of the screen area. One positive feature of the filter design was that flow continues to the process during the cleaning cycle, eliminating the need for redundant filters.
Clean-ability of the screens has long been an issue for self-cleaning mechanical filters. The basic premise for cleaning the screen relies upon the differential pressure between the water supply pressure and the exhaust valve discharge atmospheric pressure. Older designs typically included slotted nozzles running the length of the screen incorporating the same self-cleaning philosophy. The suction velocity created at the nozzle determines the efficiency of the cleaning device. With a slotted cleaning device, the area of the nozzle is so large the effective velocity is very low. Amiad Filtration Systems' patented design combines a small nozzle diameter with the geared scanner shaft that provides high suction velocity with a controlled path to clean 100% of the screen surface. This design provides suction velocities 3-5 times greater than other designs commercially available. As the filtration degree is reduced, the energy required to remove silt entrained in the screen increases. For applications above 50 micron the manufacturer requires inlet pressures of 30-35 psi. However, fine screen systems require 45-50 psi to ensure proper cleaning.
Additional benefits of the concentrated suction scanner cleaning device are the low backwash water requirements. Typically, the filter will utilize <1% of the total flow for backwash. This compares favorably to 5-10% backwash on media systems. In applications where extremely low backwash water volumes are required, Amiad has developed the spring loaded suction scanner nozzle. This advanced design has reduced backwash water volume to < 0.5% of the total processed flow.
While the above application is focused on service cooling and mechanical seal water, automatic self-cleaning filters are successfully used in cooling tower systems, plant feedwater, condensate return filtration; essentially any applications where suspended solids create an issue with plant maintenance and operations.
SOURCE: Amiad Water Systems